1. Field of the Invention
This invention relates to a vertical speed indicator, particularly suited for use in aircraft, that is relatively inexpensive, accurate and has a sufficiently rapid response time.
2. Description of the Prior Art
In modern day aviation, aircraft pilots need an instrument that indicates whether an aircraft is ascending or descending and the vertical speed at which this occurs. While such an instrument provides valuable assistance to a pilot during flight under visual flight rule (VFR) conditions where visibility is usually good, this instrument is essential whenever the pilot flies under instrument conditions (instrument flight rules--IFR) where visibility may be marginal or even non-existent.
In most aircraft and specifically for purposes of redundancy, vertical speed information is provided through two independent flight instruments: an artificial horizon and a vertical speed indicator (VSI). The artificial horizon, in its simplest form, is a gyroscopic based device that provides the pilot with visual information regarding the attitude (pitch and roll) of his aircraft with respect to the earth's horizon. The artificial horizon contains an horizon indicator which typically forms part of a gimballed gyroscope and generally remains in a fixed position with respect to the earth's horizon while the aircraft is in motion. The horizon indicator is frequently divided into two areas: illustratively a light blue upper area representing the earth's atmosphere and a differently colored, illustratively black, lower area representing the earth's surface with the horizon being the border between both areas. A marker representing an aircraft itself is secured to the instrument panel of the aircraft and is located in front of the horizon indicator. Inasmuch as the horizon indicator freely moves with respect to the marker, the indicator visually depicts the position of the horizon with respect to the aircraft at any given time. If the aircraft is climbing, then the horizon indicator rotates downward such that the marker is in front of the blue area. Similarly, if the aircraft is descending, then the horizon rotates upward such that the marker is in front of the other area. While the horizon indicator provides valuable attitude information, it only provides a coarse pitch indication typically in degrees up or down from the horizon. Unfortunately, the artificial horizon does not provide a numeric value of the vertical speed of the aircraft.
To obtain a numeric value of vertical speed, a pilot must turn to the VSI. The VSI is not a gyroscopic device. Instead, most VSIs that are in use today are rather simple mechanical devices that essentially rely on an aneroid chamber that has a diaphragm connected to an indicating needle. Specifically, a static line is connected to one end of the chamber through a relatively small bleed hole, thin tube or capillary that provides a relatively high pneumatic resistance. The remainder of the chamber is airtight. Static pressure surrounds the outside of the chamber. As a result of this arrangement, the pressure of the air that impinges on the outside of the chamber changes much faster than does the pressure of the air situated within the chamber. The resulting differential pressure between the static pressure and the chamber pressure is directly related to vertical speed. To measure this pressure, the moveable diaphragm is frequently employed as one wall of the chamber and is connected through a mechanical coupling, such as an arrangement of gears and levers, to an indicating needle set against a zero centered gauge marked in units of feet per minute (fpm). Now, in use, whenever an aircraft is in level flight, the air pressure in the chamber equals the air pressure in the static line. As such, the moveable diaphragm remains in its neutral position, thereby causing the VSI to indicate a vertical speed of zero fpm. Alternatively, if the aircraft is ascending, then the pressure in the static line correspondingly decreases but the pressure in the aneroid chamber, due to the limited amount of air that can pass through the small bleed hole at any instant, decreases less rapidly than that in the static line. As such, the relatively high pressure in the chamber causes the chamber to expand thereby distending the flexible diaphragm outward from its neutral point and away from the chamber. This movement, amplified by the mechanical coupling, causes the needle to indicate an upward vertical speed. Now, if the aircraft is descending, then the static pressure increases while the pressure inside the chamber does not increase as rapidly due to the existence of the small bleed hole. Consequently, the relatively low pressure in the chamber causes the chamber to contract thereby distending the flexible diaphragm inward from its neutral point towards the chamber. This movement, amplified by the mechanical coupling, causes the needle to indicate an downward vertical speed. Clearly, if the aircraft maintains a steady climb or descent, i.e. at a constant vertical speed, then the pressure difference between the inside of the chamber and the static line stabilizes at a corresponding value thereby causing the needle to indicate a constant vertical speed.
In practice, a period of time typically ranging between 0.5 to 1.5 seconds is required for the chamber pressure to stabilize in aneroid based VSIs. This means that for between 0.5 to 1.5 seconds after a climb or dive has ceased, an based VSI will still be indicating non-zero vertical movement. Hence, if a pilot relied solely on a VSI reading, then the time lag would cause the pilot to over control the aircraft thereby causing it to oscillate about a desired altitude. As such, pilots realize that they can not rely solely on the reading provided by an aneroid based VSI to maintain their aircraft in level flight. Specifically, experienced pilots, typically those with several thousands of flight hours, encounter great difficulty in keeping their aircraft perfectly level during certain maneuvers, such as illustratively procedure turns, based on a VSI reading alone.
Moreover, quite apart from any pressure differences occurring between the static pressure and the chamber pressure, the flexible diaphragm can move simply as the result of gravitational and other forces (e.g. centripetal and centrifugal) exerted on VSI whenever the aircraft is making a turn, such as a sharp turn, or during a dive or climb particularly while the aircraft is accelerating. As a result, the force induced movement of the diaphragm erroneously corrupts the vertical speed indication produced by the VSI. Additional errors occur, particularly in level flight, due to essentially random noise-like variations in the static line pressure. These variations produce small erratic pressure differences within the VSI and hence erratic vertical speed indications during level flight. Although, over time, an experienced pilot learns to recognize those situations when a VSI reading may be erroneous and then mentally correct the reading accordingly, these acts do increase the workload of the pilot and can become burdensome, particularly during certain maneuvers, when the pilot can least afford an additional task.
Hence, in situations where loss of altitude is to be assiduously avoided, pilots can not rely on the VSI to provide sufficiently accurate and responsive vertical speed information. Unfortunately, as noted, the only other available instrument that provides pitch information is the artificial horizon, but it only provides a coarse indication of vertical speed which is often of insufficient resolution to be of much use in these situations.
Furthermore, over a prolonged period of time, the aneroid chamber in a VSI may develop leaks which reduce the accuracy of the VSI. As such, during the life of an aircraft, aneroid type VSIs may need to be replaced from time to time. Since the aneroid chamber and its mechanical coupling are expensive, initial purchase of such a VSI or its replacement can be rather costly.
Consequently, a number of attempts have occurred in the art to replace the mechanical components, including the aneroid chamber and the mechanical coupling, with a suitable inexpensive electronic device having sufficient resolution and accuracy and a sufficiently rapid response time. For various reasons, these attempts have generally proven to be unsuccessful.
One such attempt, which is directed at producing a VSI that has a relatively fast response but little variation at level flight conditions, is disclosed in U.S. Pat. No. 3,374,671 (issued to S. B. Williams et al on Mar. 26, 1968 and hereinafter referred to as the U.S. Pat. No. '671 Williams et al). Here, a vertical speed indicator (referred to as a vertical rate sensor) is described which utilizes a capacitive differential pressure transducer and an electrical compensating circuit. The transducer is formed of a dielectric housing that is bisected by a metallic diaphragm. The diaphragm is situated between a pair of symmetrically spaced apart conductive plates (electrodes) that are attached to opposite internal walls of the housing. A separate air cavity is formed between each plate and the diaphragm. One cavity is directly connected to atmospheric pressure. The other cavity is connected to the atmospheric pressure through a capillary tube, i.e. having a relatively small inside diameter and thereby providing a relatively high pneumatic resistance. Differential pressure within the housing caused by pressure changes attributable to vertical speed causes the diaphragm to move. As such, the transducer forms a differential capacitor. To detect the direction and magnitude of the movement of the diaphragm and of the vertical speed of the aircraft, both plates are magnetically coupled to an identical source of an alternating current (AC) exciting signal. This signal is also magnetically coupled to two opposite terminals of a bridge rectifier. The voltage appearing between the diaphragm and ground is amplified, then rectified using the bridge rectifier which serves as a synchronous phase detector, and thereafter applied through a suitable filter and compensating circuit as an output signal.
As long as the pressures within both cavities are the same, i.e. indicative of level flight, then the diaphragm does not move. Consequently, the voltage on the diaphragm is essentially zero. As the aircraft changes altitude, then the diaphragm will deflect away from that cavity that has a relatively high pressure and towards the other cavity that has a relatively low pressure, thereby moving towards one electrode or the other depending upon whether the aircraft is ascending or descending. In this case, the electrodes of the transducer in conjunction with the diaphragm will provide unequal amounts of capacitance which will unbalance the bridge. The phase and magnitude of the voltage appearing on the diaphragm, after being appropriately scaled, indicates the direction and magnitude of the vertical speed of the aircraft. Now, to increase the sensitivity of the transducer, the cavity of the transducer that is connected to the capillary tube is also connected to an additional chamber that provides a high acoustic capacitance. This acoustic capacitance imparts a relatively long time constant to this cavity. To compensate for this added delay, the filter and compensating circuit contains a parallel resistive-capacitive (R-C) circuit that has a pre-defined leading phase angle.
The prior art device disclosed in the U.S. Pat. No. '671 Williams et al and similar devices that rely on using a moving metallic diaphragm to implement a differential capacitor contain certain drawbacks. As such, these devices have failed to live up to expectations. First, the metallic diaphragm is subject to both gravitational forces as well as centrifugal and centripetal forces while the aircraft is in motion. These forces, singly or in combination, will tend to erroneously deflect the diaphragm and, as such, cause an erroneous vertical speed reading to occur. Second, the capacitances provided by such a device often vary for reasons that are unrelated to the deflection of the diaphragm. For example, the capacitance between the diaphragm and either electrode is a function of the spacing therebetween as well as the moisture content of the air existing therein. Since the transducer will be passing from one altitude to another, each of which may have a different moisture content, and the cavities exhaust and fill at different rates, the air contained in both cavities may have different moisture contents. Consequently, the capacitance associated with each cavity will contain an erroneous variation due to the moisture content of the air contained therein which is both unknown to the pilot and generally not easily correctable thereby corrupting the vertical speed information provided by the device.
Another prior art version of a VSI, having improved sensitivity and shortened response time, is discussed in U.S. Pat. No. 3,703,828 (issued to E. R. Bullard et al on Nov. 28, 1972 and hereinafter referred to as the U.S. Pat. No. '828 Bullard et al). This patent recognizes that the deflection of a diaphragm, such as that used in the U.S. Pat. No. '671 Williams et al, is inversely proportional, and hence non-linearly related, to the change in capacitance produced thereby. Consequently, to provide a linear relationship between diaphragm deflection and capacitance and hence vertical speed, the U.S. Pat. No. '828 Bullard et al discloses a VSI that uses dual diaphragms to eliminate the non-linear relationship between diaphragm deflection and capacitance. Unfortunately, this version, which uses metallic diaphragms and has a cavity that fills and exhausts with atmospheric air, is subject to the same drawbacks discussed above for the device disclosed in the '671 Williams patent.
Another attempt at providing an altitude change indicator, here for use in a relatively slow moving model airplane, that relies on an absolute pressure transducer, specifically using a moveable membrane, is shown in U.S. Pat. No. 4,238,791 (issued to R. Wiebalck on Dec. 9, 1980). Here, the moveable membrane is stretched across the top of a sealed air chamber and attached to a low inertia light shield that serves to partially block or unblock light from emitted from a photo-diode as the altitude of the aircraft changes and the membrane correspondingly moves. The amount of light travelling past the shield is electrically differentiated and amplified to provide a signal that changes in relation to changes in aircraft altitude. Although the membrane and light shield can be fabricated from materials having a relatively small amount of inertia, these elements are nonetheless subject to erroneous movement caused by acceleration and other forces resulting from movement of a fast moving aircraft which, when differentiated, can produce significantly erroneous readings. A further prior art device that relies on the use of a moveable diaphragm (or separator) and is particularly prone to providing erroneous readings due to force induced movement of the diaphragm is disclosed in U.S. Pat. No. 3,321,968 (issued to E. S. Joline on May 30, 1967).
Another attempt at providing a VSI indicator, specifically a VSI that indicates vertical speed of an aircraft, e.g. a glider, that is attributable to vertical air movement and compensated to eliminate control surface induced aircraft movement, is disclosed in U.S. Pat. No. 4,086,810 (issued to R. H. Ball on May 2, 1978). This indicator relies on additively combining the outputs of a first absolute pressure transducer that senses altitude variations with that of a second absolute pressure transducer that senses airspeed variations to provide a resultant signal and then differentiating and then appropriately amplifying the resultant signal to provide an output signal that drives an indicator to depict rate of climb (vertical speed) of the aircraft. Unfortunately, differentiation is very sensitive to input noise. Since the transducer outputs are likely to contain noise components, then the rate of climb output signal is likely to contain significant amounts of noise. While input and/or output filtering could be added, this filtering disadvantageously slows the response time of the circuit.
Lastly, a further attempt at providing a VSI having an increased sensitivity and a shortened response time is disclosed in U.S. Pat. No. 3,769,827 (issued to A. G. Moore on Nov. 6, 1973). The VSI here utilizes two piezoelectric elements bonded to either side of a metallic diaphragm (thereby forming a "bimorph" element) that is mounted on a thin circumferential annularly shaped flexure situated within and bisecting a chamber. One side of the chamber is sealed; while, the other side is connected to a static line of an aircraft. As the altitude of the aircraft changes, the diaphragm flexes thereby causing each piezoelectric element to generate a corresponding voltage. The voltage generated by each piezoelectric element is then applied to an amplifier which provides an output responsive to the difference therebetween and hence to the vertical speed of the aircraft. Inasmuch as this device, not unlike those discussed above, relies on flexure of a metallic diaphragm, this device is also susceptible to producing erroneous readings as the result of centripetal and other forces exerted on the bimorph due to movement of the aircraft.
Therefore, a need exists in the art to provide an relatively inexpensive, accurate electronic VSI that is substantially immune to various forces (e.g. gravitational, centripetal and centrifugal) that will likely occur in the aircraft and which provides a relatively rapid response in comparison to vertical speed indicators known in the art.